The present invention relates to a variable phase drive mechanism for providing drive from an engine crankshaft to two sets of cams.
There have previously been proposed variable phase drive mechanisms that use hydraulic pressure to couple the drive and driven members to one another.
U.S. Pat. No. 5,002,023 uses a conventional pair of piston/cylinder units, or jacks, to couple a drive member (a sprocket) to a driven member (a camshaft). Because the connections of the jacks to the drive and driven members must allow relative pivoting, such a design is complex and bulky and makes it difficult to establish hydraulic connections with the working chambers of the jacks. In the case of an engine with two camshafts, this patent proposes mounting such a variable phase drive mechanism on the drive sprocket of each of the camshafts to allow their phases to be varied independently of one another relative to the engine crankshaft.
EP-0 924 393 and GB-2 319 071 use an alternative form of hydraulic coupling in which an annular space is provided between concentric drive and driven members. The space is divided into segment-shaped or arcuate variable volume working chambers by means of a first set of vanes extending radially inwards from the inner surface of the drive member and a second set of vanes that extend outwards from the outer surface of the driven member. As hydraulic fluid is admitted into and expelled from the various chambers, the vanes rotate relative to one another and thereby vary the relative angular position of the drive and driven members. Hydraulic couplings that use radial vanes to apply a tangentially acting force rather than a linear acting force will herein be referred to as vane-type couplings.
According to the present invention, there is provided a variable phase drive mechanism for providing drive from an engine crankshaft to two sets of cams, the drive mechanism comprising a drive member connectable for rotation with the engine crankshaft and two driven members each connectable for rotation with a respective one of the two sets of cams, wherein the drive and driven members are all mounted for rotation about a common axis and the driven members are coupled for rotation with the drive member by means of vane-type hydraulic couplings.
In prior art proposals, separate drive mechanism were needed to permit the phases of two camshafts to be varied independently of one another relative to the engine crankshaft. By providing two driven members that rotate about the same axis as the drive member, the invention permits a single drive mechanism to be used for both camshafts, thereby providing a significant cost saving.
The invention also offers a significant reduction in the size of the mechanism as the entire phase shifting mechanism can be accommodated within the space normally occupied by a conventional cam drive pulley or sprocket.
It is possible for the two sets of cams to be rotatable about the same axis as one another, the engine having a camshaft assembly in which the first set of cams is mounted on an outer tube and the second set of cams is fast in rotation with an inner shaft mounted concentrically within and rotatable relative to the outer tube.
Alternatively, the two sets of cams may be fixed cams on two separate camshafts each rotatable with a respective one of the driven members. In this case, one of the driven members may be directly connected to one of the camshafts while the other may be connected using a chain, a toothed belt or a gear train.
The hydraulic connection between the drive member and each of the driven members may comprise at least one arcuate cavity defined between the members and a radial vane projecting from one of the members into the arcuate cavity to divide the cavity into two variable volume working chambers, the pressure in which working chambers acts on the opposite sides of the radial vane.
As a further possibility, an arcuate cavity defined between the members may be divided into three working chambers by two radial vanes each fast in rotation with a respective one of the members. In this case, the pressures in the three working chambers may varied to set the desired angular position of each of the vanes within the cavity independently of the other.
Regardless of whether the arcuate cavities are divided into two or three working chambers, it is possible to arrange for all the cavities to intersect a common plane normal to the rotational axis of the members. This enables the mechanism to be very compact as it avoids the vane-type couplings having to be staggered along the axis of the mechanism.